1. Field
Embodiments of the present invention generally relate to methods and apparatus for preventing particle contamination. Particularly, embodiments of the present invention provide methods and apparatus for protecting masks and/or substrates during lithography.
2. Description of the Related Art
As the trend continues to reduce the size of semiconductor devices, optical lithography using conventional transmission masks, such as chrome on glass (COG) or phase shift (PSM) masks, will no longer suffice as a viable technique for printing advanced devices on semiconductor wafers. Transmission lithography has been extended to ever shorter wavelengths, down to 157 nm in the far ultraviolet (UV), in order to reduce the size of device features. However, the still shorter wavelengths necessary for printing even smaller device structures are readily absorbed in transmission materials. Alternative technological candidates to replace optical lithography include: electron projection lithography (EPL) and an all-reflective technology called extreme ultra-violet lithography (EUVL).
Generally, masks used in production today employ a pellicle to protect the mask surface from particulate contamination. The pellicle is a relatively inexpensive, thin, transparent, flexible sheet, which is stretched above and not touching the surface of the mask. Pellicles provide a functional and economic solution to particulate contamination by mechanically separating particles from the mask surface. The mask is transported and used for lithographic exposure with the pellicle in place. When a mask is used for exposure, with the pellicle in position above the mask, only the details of the mask's focal plane itself are printed. Particulate material located on the pellicle surface is maintained outside of the focal plane of projection. As a result, particulate material is not printed. When the pellicle eventually becomes damaged or too dirty to use, the mask is removed to a workshop, and the pellicle is replaced.
However, in EUV lithography, conventional pellicles can not be used to protect masks during process because all materials are opaque to EUV light. Even though, masks may be protected by pellicles when not in use. Masks must be exposed during lithography. Additionally, the minimum particle size to be removed has decreased with the decrease of the wavelength of the radiation source. For example, in EUV lithography for 65 nm features, particles as small as 52 nm must be removed.
Therefore, there is a need for apparatus and methods for protecting masks during lithography.